FRAUNHOFER, frown'hō-fĕr, Josephvon, German mathematician: b. Straubing,
Bavaria, 6 March 1787; d. Munich, 7 June 1826.
In 1799 he was placed with a looking-glass
maker and glass-grinder at Munich. After
various vicissitudes he received an appointment
as optician in the mathematical and mechanical
institute of Reichenbach and Utzschneider at
Benedictbeuren, and in 1809 the mechanical part
of the optical institute was chiefly under his
direction. In the same year he became one of
the members of the firm under which the
business was conducted, and in 1818 its director.
In 1823, after it was moved to Munich,
Fraunhofer became a member of the Academy of
Science, its conservator of physics and was
made a member of the nobility in 1824. One
of the most difficult operations of practical
optics was to polish the spherical surfaces of
large object-glasses accurately. Fraunhofer
invented a machine which obviated this difficulty,
and rendered the surface more accurate than it
was left by the grinding. He invented also
other grinding and polishing machines, and
introduced many improvements into the
manufacture of the different kinds of glass used
for optical instruments, and which he found to
be always injured by flaws and irregularities of
various sorts. In 1811 he constructed a new
kind of furnace, and on the second occasion
when he melted a large quantity found that he
could produce flint-glass, which, taken from the
bottom of a vessel containing two hundred-weight
of glass, had the same refractive power
as glass taken from the surface. He found that
the English crown-glass and the German table-glass
both contained defects occasioning irregular
refraction. In the thicker and larger glasses
there would be more of such defects, so that
in larger telescopes this kind of glass would
not be fit for object-glasses. Fraunhofer therefore
made his own crown-glass. The cause
which had hitherto prevented the accurate
determination of the power of a given medium
to refract the rays of light and separate the
different colors which they contain was chiefly
the circumstance that the colors of the spectrum
have no precise limits, and that the transition
from one to another is gradual and not
immediate; hence, the angle of refraction cannot
in the case of large spectra be measured within
10 feet or 15 feet. To obviate this, Fraunhofer
made a series of experiments for the purpose
of producing homogeneous light artificially, and
unable to effect his object in a direct way, he
did so by means of lamps and prisms. In the
course of these experiments he discovered that
bright fixed line which appears in the orange
color of the spectrum when it is produced by
the light of fire. This line enabled him afterward
to determine the absolute power of refraction
in different substances. Experiments to
ascertain whether the solar spectrum contains
the same bright line in the orange as that
produced by the light of fire led him to the
discovery of the innumerable dark fixed lines in
the solar spectrum, consisting of perfectly
homogeneous colors and now bearing his name.
The importance of this discovery can scarcely
be overestimated. It led to the invention and
use of the spectroscope (q.v.), to the science
of spectroscopy (q.v.), and to all our present
knowledge of solar and stellar chemistry.
Fraunhofer also made a variety of other
important discoveries and inventions. He made
the great refracting telescope for Dorpat His
writings were edited by Lommel as
‘Gesammelte Schriften’ (Munich 1888). Some of his
writings were translated into English by J. S.
Ames and published as the second volume of
‘Harper's Scientific Memoirs’ under the title
‘Prismatic and Diffraction Spectra’ (New
York 1898). Consult Anon., ‘Life of Fraunhofer’
(in American Journal of Science andArt, Vol. XVI, p. 304, New Haven 1829);
Voit, ‘Joseph Fraunhofer’ (Munich 1887).